The invention relates to fuel injectors suitable for use in a combustor of a gas turbine engine and in particular fuel injectors suitable for use in lean burn combustors of a gas turbine engine.
With reference to FIG. 1, a ducted fan gas turbine engine generally indicated at 10 comprises, in axial flow series, an air intake 1, a propulsive fan 2, an intermediate pressure compressor 3, a high pressure compressor 4, combustion equipment 5, a high pressure turbine 6, an intermediate pressure turbine 7, a low pressure turbine 8 and an exhaust nozzle 9.
Air entering the air intake 1 is accelerated by the fan 2 to produce two air flows, a first air flow into the intermediate pressure compressor 3 and a second air flow that passes over the outer surface of the engine casing 12 and which provides propulsive thrust. The intermediate pressure compressor 3 compresses the air flow directed into it before delivering the air to the high pressure compressor 4 where further compression takes place.
Compressed air exhausted from the high pressure compressor 4 is directed into the combustion equipment 5, where it is mixed with fuel and the mixture combusted. The resultant hot combustion products expand through and thereby drive the high 6, intermediate 7 and low pressure 8 turbines before being exhausted through the nozzle 9 to provide additional propulsive thrust. The high, intermediate and low pressure turbines respectively drive the high and intermediate pressure compressors and the fan by suitable interconnecting shafts.
The combustion equipment comprises one or more combustion chambers and fuel and air is injected into the, or each, combustion chamber through one or more fuel injectors. Where the combustion chamber is an annular combustion chamber a number of fuel injectors are circumferentially spaced along an upstream bulkhead of the combustion chamber.
Whilst the majority of the air flowing through a gas turbine engine passes through the combustion it is typically only a small proportion that passes through the fuel injector itself. The small proportion, around 10 to 15% of the total air entering the combustor, travels relatively slowly and provides a primary combustion point for the fuel injected and maintains the continuous combustion required for operation of a gas turbine. The remaining air enters the combustion chamber enters downstream of this primary zone and both dilutes the hot air caused by combustion of the fuel and provides cooling to protect the walls of the combustor.
NOx is a pollutant that may be formed at high temperatures as a by-product of the combustion process. To avoid production of such a pollutant, more recent “lean burn” fuel injectors propose increasing the flow of air into the combustor through the injectors to around 70% of the total airflow entering the combustor. These injectors typically have a pilot injector located around a central axis and a coaxial main injector. The pilot injector is continually fed with fuel and a specified percentage of air. The main injector is fed with a continual flow of air and an intermittent flow of fuel for times when high engine power is required.
The air steam within both the pilot and main injectors is induced to swirl by the provision of swirl vanes that extend radially between an inner hub and an outer, circumferentially extending, periphery.
The flow of air through the main injector is generally larger than that of the pilot injector. Fuel is fed to an annular outlet within the main injector that allows the fuel to flow in an annular film along an atomiser filmer lip. The annular film of liquid fuel is entrained within the much more rapidly flowing and swirling air stream. The air streams cause the annular film of fluid to be atomised into small droplets dispersed within the stream.
At high volumetric air flows, typical of lean burn injectors, non uniform air flow from the swirlers affects the flow quality of the air on the filmer lip. This in turn affects the atomisation performance of the air flow on the fuel and can lead to higher NOx production than desired.
It is an object of the present invention to seek to address these and other problems and to seek to provide an improved fuel injector.